Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, and various signal processing circuits.
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual images for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
FIG. 1 illustrates electronic device 50 having a chip carrier substrate or printed circuit board (PCB) 52 with a plurality of semiconductor packages mounted on a surface of the PCB. Electronic device 50 can have one type of semiconductor package, or multiple types of semiconductor packages, depending on the application. Different types of semiconductor packages are shown in FIG. 1 for purposes of illustration.
Electronic device 50 can be a stand-alone system that uses the semiconductor packages to perform one or more electrical functions. Alternatively, electronic device 50 can be a subcomponent of a larger system. For example, electronic device 50 can be part of a tablet computer, mobile phone, digital camera, television, or other electronic device. Electronic device 50 can also be a graphics card, network interface card, or other expansion card that is inserted into a personal computer. The semiconductor packages can include microprocessors, memories, application specific integrated circuits (ASIC), programmable logic circuits, analog circuits, radio frequency (RF) circuits, discrete devices, or other semiconductor die or electrical components.
For the purpose of illustration, several types of first level packaging, including bond wire package 56 and flipchip 58, are shown on PCB 52. Additionally, several types of second level packaging, including ball grid array (BGA) 60, bump chip carrier (BCC) 62, land grid array (LGA) 66, multi-chip module (MCM) 68, quad flat non-leaded package (QFN) 70, quad flat package 72, embedded wafer level ball grid array (eWLB) 74, and wafer level chip scale package (WLCSP) 76 are shown mounted on PCB 52. Depending upon the system requirements, any combination of semiconductor packages, configured with any combination of first and second level packaging styles, as well as other electronic components, can be connected to PCB 52.
A manufacturer of electronic device 50 provides a power signal to the electronic device which is used to power the semiconductor packages and other devices disposed on PCB 52. In many cases, the provided power signal is at a different voltage potential than the voltage requirement of the individual semiconductor packages. The manufacturer provides a power converter circuit on PCB 52 to generate a steady direct current (DC) voltage signal at a voltage potential usable by the individual semiconductor packages. Switch-mode power supplies (SMPS) are commonly used due to efficiency advantages.
An SMPS switches an input power signal on and off repeatedly using a primary MOSFET to create a relatively high-frequency power signal. The switched power signal is routed through a transformer or inductor, and then rectified and filtered to create a steady DC power signal. Some prior art power supplies use a conventional diode to perform rectification. However, a diode has a substantially fixed voltage potential between the terminals of the diode, which causes relatively high power losses as output current of the SMPS increases.
Synchronous rectification is used as an alternative to diode rectification in many SMPS topologies. Synchronous rectification uses a synchronous rectification (SR) MOSFET, enhancement mode gallium arsenide transistor, or other electronically controlled switch to enable secondary current when secondary current is flowing in the appropriate direction. When turned on, an SR MOSFET has a relatively low resistance that is substantially constant, and generally produces a significantly lower voltage drop across the SR MOSFET than a diode. Accordingly, synchronous rectification is generally more efficient than using a diode for rectification.
An SR controller turns the SR MOSFET on and off based on the voltage potential between the drain and source terminals of the SR MOSFET. When the drain-to-source voltage of the SR MOSFET drops below zero volts, the SR controller turns on the SR MOSFET. However, parasitic ringing during and after primary or SR MOSFET switching can cause the voltage across the SR MOSFET to cross the zero volt threshold several times during one SMPS switching period. If no system for ignoring the ringing is implemented, the SR controller may switch the SR MOSFET on or off at incorrect times. Turning on the SR MOSFET when the SR MOSFET should not be conducting results in current from the voltage output being routed to a ground node. Turning off the SR MOSFET when the SR MOSFET should be conducting results in current conducting through the body diode of the SR MOSFET, which has a larger voltage drop. In either case, the overall efficiency of the system is reduced.
Some SR controllers implement a minimum off-time and minimum on-time for the SR MOSFET. The minimum off-time is set larger than the expected drain-to-source voltage ringing time so that transitions across the zero volt threshold are ignored until the ringing is reduced to a safe magnitude. A problem exists with flyback type converters that operate on a variable output voltage. When output voltage is reduced, the primary MOSFET on-time may be reduced as well. If the on-time of the primary MOSFET is reduced below the minimum off-time for the SR MOSFET as set in the SR controller, the SR controller misses signal transitions that should cause the SR MOSFET to turn on, but that occur within the minimum off-time. The SR controller does not operate properly, and efficiency is significantly reduced.